1. Technical Field
The present invention relates to a liquid ejecting apparatus which is represented by a fax machine or a printer.
2. Related Art
Hereinafter, an ink jet printer will be described as an example of a liquid ejecting apparatus. The ink jet printer includes an ink jet printing head and a support member (also called a platen) that is disposed at a position facing the ink jet printing head. A distance between the ink jet printing head and a printing sheet is defined by supporting the printing sheet using the support member.
Here, in ink jet printers in recent years, there has been a tendency for the size of an ink droplet to become smaller in order to further improve the printing quality. For example, the size of the ink droplet has become smaller to be about several pl. For this reason, the mass of the ink droplet is extremely small, and even when the ink droplet is ejected from the ink jet printing head onto the printing sheet, a part of the ink droplet does not land on the printing sheet, and floats in the form of mist, which causes various problems in ink jet printers. Further, in a so-called margin-less printing process in which the printing process is performed without any margin on four sides of the printing sheet, since the ink droplet is ejected even onto an area offset from the end of the printing sheet, the above-described mist floating phenomenon occurs more prominently.
Therefore, a technology has been proposed by JP-A-2007-118321 and JP-A-2007-118318, in which an ink jet printing head, a printing sheet, and a support member are set to have a difference in potential to generate an electric field, and a coulomb force is applied to an ink droplet to pull the ink droplet toward the printing sheet.
However, in recent years, ink jet printers have adopted a fixed ink jet printing head which is called a line head that is designed not to perform a scanning (moving) operation, but to perform a printing process with extremely high throughput. In this kind of ink jet printer, the printing sheet is transported at extremely high speed via a sheet transporting path inside the ink jet printer.
However, the following problems arose as the transportation speed of the printing sheet became faster. That is, paper dust generated when cutting a large original paper sheet adheres to the end (edge) of the printing sheet, but when a difference in potential is not adjusted in three components (hereinafter, generally referred to as “printing section constituting components”) of the printing sheet, the support member (platen), and the ink jet printing head, the paper dust adhering to the printing sheet flies toward the ink jet printing head to thereby adhere thereto due to an electric field generated between the printing section constituting components. In particular, when the printing sheet is transported at high speed, vibration or impact generated during the transportation of the printing sheet becomes larger. For this reason, the paper dust flying phenomenon occurs more prominently.
Further, in accordance with friction between the printing sheets accommodated in a sheet cassette or slidable contacting between the printing sheet and the constituting component (for example, an edge guide, a transportation roller, or the like) of the sheet transporting path, friction charging or peeling charging becomes more prominent, that is, the printing section constituting components are more prominently charged. As a result, the electric field formed between the printing section constituting components becomes stronger, and charging of paper dust becomes stronger. For this reason, a coulomb force applied to the paper dust becomes larger, and the adhering of the paper dust to the ink jet printing head becomes more prominent.
Furthermore, even when the paper dust itself is not charged, if the flying paper dust is disposed within the electric field, biased charges occur in the paper dust due to dielectric polarization or electrostatic induction, thereby pulling the paper dust toward the ink jet printing head.
FIG. 7 is a diagram illustrating such problems, where the reference numeral 160 indicates an ink jet printing head, the reference numeral 160a indicates a nozzle plate, the reference numeral 170 indicates a support member (platen), and the reference numeral 170a indicates a rib formed on the support member 170. In addition, the reference numeral P indicates a printing sheet, the reference numeral Pe indicates a sheet end, and the reference numeral d indicates paper dust. Further, the circled “+” and “−” indicate charge polarities.
The printing sheet P is neutralized by a neutralization brush or the like, and hence the paper dust d adhering to the printing sheet P is not charged. However, as an example, when the nozzle plate 160a is charged to be positive, and the support member 170 is charged to be negative as shown in the enlarged view of the paper dust d, the paper dust d on the side of the nozzle plate takes negative charges, and the paper dust d on the side of the support member takes positive charges due to charge polarization (when the paper dust d has a property of a dielectric body) or electrostatic induction (when the paper dust d has a property of a conductor). Accordingly, the paper dust d is pulled toward any one of the nozzle plate 160a and the support member 170.
Then, when the paper dust adheres to the ink jet printing head, the paper dust directly blocks a nozzle opening, or the paper dust moves to the nozzle opening when a nozzle surface is cleaned (wiped), thereby causing dot omission.
In addition to the problem that the paper dust physically blocks the nozzle opening, a loading material such as calcium carbonate forming the paper dust causes a reaction with moisture to thereby be thickened and to thereby deteriorate vibration of a meniscus of the nozzle opening, which may disturb the ejection of the ink droplet. Accordingly, it is very important to prevent the paper dust from adhering to the ink jet printing head in order to obtain the appropriate printing quality in the ink jet printer.
As described above, JP-A-2007-118321 and JP-A-2007-118318 disclose a technology in which the ink jet printing head, the printing sheet, and the support member (the printing section constituting components) are set to have a difference in potential to generate the electric field, and the coulomb force is applied to the ink droplet to pull the ink droplet toward the printing sheet. Accordingly, assuming that the paper dust and the ink droplet are on the same line, it is thought that the paper dust may be prevented from adhering to the ink jet printing head in such a manner that the paper dust is pulled toward the printing sheet by controlling the electric field.
However, loading material and cellulose fiber forming the paper dust may be easily charged to any one of positive and negative polarities in the triboelectric series. Accordingly, even when there is an attempt to prevent the paper dust from flying toward the ink jet printing head by forming an electric field in a predetermined direction between the printing section constituting components, it is not possible to prevent the paper dust charged to the opposite polarity from flying toward the ink jet printing head.
Further, JP-A-2003-165230 discloses a printing apparatus in which an air duct is provided around a nozzle plate for the purpose of preventing paper dust or the like from adhering to the periphery of a nozzle portion of an ink jet printing head, and moist air is sprayed from the air duct in a printing mode and a printing standby mode. However, in this configuration, since the configuration is complex, there are concerns that the size and the cost of the printing apparatus increase, and that the paper dust unexpectedly adheres to the printing head due to the air stream.
Furthermore, JP-A-2008-213255 discloses a technology in which a paper dust collecting member having a charging property collects the paper dust. However, in the technology, the paper dust cannot be efficiently collected due to the problem involving the above-described opposite polarities, and another problem arises in that the paper dust accumulated on the paper dust collecting member needs to be treated (removed). In particular, when a large amount of paper dust is accumulated, the paper dust may scatter in the peripheral area thereof due to slight vibration or impact, thereby causing a problem that performance cannot be maintained over the long term.